THE IMPACTS OF PRESCRIBED BURNING ON PHOSPHORUS MOBILITY IN SALT MARSH SURFACE SOILS

Salt marshes provide the important ecosystem service of improving water quality by filtering nutrients like phosphorus (P) via storage in biomass and soils. Plant-available P is necessary for plant growth, but excess mobile P can cause eutrophication by increasing nutrient export to estuarine waters. This P storage service is hypothesized to be impacted by prescribed burning, which is a common method for invasive grass (Phragmites) removal in salt marshes on the East Coast of the United States. While invasive Phragmites may store more P than the native vegetation, biochar produced during burning could help mitigate some of the ecosystem services lost during invasive Phragmites removal, via the sorption of P forms onto biochar and/or incorporating biochar-associated P into soils. The impact of prescribed burning on P mobility in salt marsh surface soils is assessed in this study by sampling three marshes with different burn histories: never burned, burned recently (<5 yr), and not burned recently (>20 yr). Sequential extractions are used to separate four organic and inorganic P pools: immediately available (in soil solution), easily exchangeable (bound to the surface of soil particles), intermediately available (Fe/Al bound), and refractory (Ca/mineral bound). P species concentrations are compared to those of black carbon, a proxy for biochar inputs. Preliminary data show with increasing black carbon concentration, organic P in the easily exchangeable P pool may increase. No significant differences were found in the inorganic P pools in relation to burn history. An increase in P concentration in the easily exchangeable organic P pool may result from the charred biomass itself, or from the sorption of organic P soil components to the biochar. Biochar from prescribed burns thus appears to increase the organic P pool, but the increased P concentrations may not be long-lived in the soils. The final results will provide insight into the biogeochemical impacts of prescribed burning and can be used by marsh managers assessing the utility of this practice for their marshes.